1. Field of the Invention
The present invention relates to fluid-fluid contacting trays and, more particularly, but not by way of limitation, to an improved fluid dispersion device and tray assembly incorporating fixed and/or floating units having deflector tabs disposed across a tray for higher efficiency operation.
2. History of Related Art
It is well known to utilize distillation columns to separate selected components from a multicomponent stream. Generally, such contact columns utilize either trays, packing, or combinations thereof. In certain years the trend has been to replace so-called “bubble caps” by sieve and valve trays in most tray column designs. Valve trays remain a favorable design.
Successful fractionation in the column is dependent upon intimate contact between heavier fluids and lighter fluids. Some contact devices, such as trays, are characterized by relatively high pressure drop and relatively high fluid hold-up. One type of contact apparatus utilizes fluid in the vapor phase to contact fluid in the liquid phase and has become popular for certain applications. Another type of contact apparatus is high-efficiency packing, which is energy efficient because it has low pressure drop and low fluid hold-up. However, these very properties at times make columns equipped with structured packing difficult to operate in a stable, consistent manner. Moreover, many applications simply require the use of trays.
Trays for fractionation columns are commonly designed in two configurations: cross-flow and counter flow. The trays generally consist of a solid tray or deck having a plurality of apertures and are installed on support rings within the column. In cross-flow trays, lighter fluid ascends through the apertures and contacts heavier fluid moving across the tray, through the “active” area thereof. In this area, the heavier fluid and the lighter fluid mix and fractionation occurs. The heavier fluid is directed onto the tray by means of a vertical channel from the tray above. This channel is referred to as the Inlet Downcomer. The heavier fluid moves across the tray and exits through a similar channel referred to as the Exit Downcomer. The location of the downcomers determines the flow pattern of the heavier fluid. If there are two Inlet Downcomers and the heavier fluid is split into two streams over each tray, it is called a two pass tray. If there is only one Inlet and one Outlet Downcomer on opposite sides of the tray, it is called a single pass tray. For two or more passes, the tray is often referred to as a Multipass Tray. The number of passes generally increases as the required (design) flow rate increases. It is the active area of the tray, however, which is of critical concern.
Addressing now select flow designs, a particularly effective tray in process columns is the sieve tray. This tray is constructed with a large number of apertures formed in the bottom surface. The apertures permit the ascending lighter fluid to flow into direct engagement with the heavier fluid that is flowing across the tray from the downcomer described above. When there is sufficient lighter-fluid flow upwardly through the tray, the heavier fluid is prevented from running downwardly through the apertures (referred to as “weeping”). A small degree of weeping is normal in trays while a larger degree of weeping is detrimental to the capacity and efficiency of a tray.
Tray efficiency is also known to be improved in sieve type trays by increasing the froth height of the heavier fluid and reducing the backflow of the heavier fluid flowing across the tray. Froth is created when lighter fluid “bubbles” percolate upwardly through the heavier fluid flowing across the tray. The suspension of the lighter fluid in the heavier fluid prolongs the fluid-fluid contact which enhances the efficiency of the process. The longer the froth is maintained and the higher the froth is established, the greater the fluid-fluid retention. Higher froth requires smaller “bubbles” formed at a sufficiently slow rate. Likewise, backflow occurs beneath the froth when circulating currents of heavier fluid are established during the heavier fluid flow across the plate. This generally forms along the lateral portions thereof. These currents carry the heavier fluid back across the tray in a manner that reduces the concentration-difference driving force for mass transfer. It is the concentration-difference between the lighter fluid and the heavier fluid which enhances the effectiveness of the fluid-fluid contact.
The concentration-difference between the lighter fluid and the heavier fluid can be effected in many ways; some reducing efficiency. For example, as operating pressure increases, the heavier fluid begins to absorb lighter fluid as it moves across a tray. This is above that normally dissolved in the heavier fluid and represents much larger amounts of lighter-fluid bubbles that are commingled or “entrained” with the heavier fluid. This lighter fluid is not firmly held and is released within the downcomer, and, in fact, the majority of said lighter fluid must be released otherwise the downcomer cannot accommodate the heavier fluid/lighter fluid mixture and will flood, thus preventing successful tower operation. This phenomena is generally deemed to occur when operating pressure is such as to produce a lighter fluid density above about 1.0 lbs/cu. ft. and typically amounts to about 10 to 20% of the lighter fluid by volume. For conventional trays, as shown below, the released lighter fluid must oppose the descending frothy lighter fluid/heavier fluid mixture flowing over the weir into the downcomer. In many cases, such opposition leads to poor tower operation and premature flooding.
When a vapor comprises the lighter fluid and a liquid comprises the heavier fluid, there are specific performance issues. Certain performance and design issues are seen in the publication “Distillation Tray Fundamentals”, M. J. Lockett, Cambridge University Press, 1986. Other examples are seen in several prior art patents, which include U.S. Pat. No. 3,338,566 issued to W. Kittel, U.S. Pat. No. 3,729,179 assigned to Fractionation Research, Inc., U.S. Pat. Nos. 3,282,576 and 4,275,021 assigned to Union Carbide Corporation and U.S. Pat. No. 4,603,022 issued to Mitsubishi Jukogyo Kabushiki Kaisha of Tokyo, Japan. A particularly relevant reference is seen in U.S. Pat. No. 4,499,035 assigned to Union Carbide Corporation that teaches a gas-liquid contacting tray with improved inlet bubbling means. A cross-flow tray of the type described above is therein shown with improved means for initiating bubble activity at the tray inlet comprising spaced apart, imperforate wall members extending substantially vertically upwardly and transverse to the liquid flow path. The structural configuration is said to promote activity over a larger tray surface than that afforded by simple perforated tray assemblies. This is accomplished in part by providing a raised region adjacent the downcomer area for facilitating gas ascension therethrough.
U.S. Pat. No. 4,550,000 assigned to Shell Oil Company teaches an apparatus for contacting a liquid with a gas in a relationship between vertically stacked trays in a tower. The apertures in a given tray are provided for the passage of gas in a manner less hampered by liquid coming from a discharge means of the next upper tray. This is provided by perforated housings secured to the tray deck beneath the downcomers for breaking up the descending liquid flow. Such advances in tray designs improve efficiency within the confines of prior art structures. Likewise, U.S. Pat. No. 4,543,219 assigned to Nippon Kayaku Kabushiki Kaisha of Tokyo, Japan teaches a baffle-tray tower. The operational parameters of high gas-liquid contact efficiency and the need for low pressure loss are set forth. Such references are useful in illustrating the need for high efficiency lighter fluid/heavier fluid contact in tray process towers. U.S. Pat. No. 4,504,426 issued to Karl T. Chuang et. al. and assigned to Atomic Energy of Canada Limited is yet another example of gas-liquid contacting apparatus.
Several prior patents have specifically addressed the tray design and the apertures in the active tray deck area itself. For example, U.S. Pat. No. 2,787,453, a 1957 patent, and U.S. Pat. No. 2,853,281, a 1958 patent, disclose directional tab-style fractionating trays that promote tray activity. By way of further example, U.S. Pat. No. 3,146,280 is a 1964 patent teaching a directional float valve. The gas is induced to discharge from the inclined valve in a predefined direction depending on the orientation of the valve in the tray deck. Such valve configurations are often designed for particular applications and flow characteristics. Tray valves with weighted sides and various shapes have thus found widespread acceptance in the prior art. A circular valve structure is shown in U.S. Pat. No. 3,287,004 while a rectangular valve structure is shown in U.S. Pat. No. 2,951,691. Both of these patents issuing to I. E. Nutter, teach specific aspects of gas-liquid contact flow utilizing tray valve systems. Such specialized designs are necessary because lighter fluid/heavier fluid flow problems must be considered for each application in which a tray is fed by a downcomer. The type of flow valve, its orientation, and the lighter-fluid flow apertures for lighter fluid-heavier fluid flow interaction are some of the issues addressed by the present invention.
Addressing specifically now the type of flow valve, its orientation, and the lighter-fluid flow apertures that currently are taught by the prior art. Attention is directed to two patents in which the inventors of the present application, Michael J. Binkley, is a co-inventor. U.S. Pat. Nos. 5,147,584 and 5,120,474, both teach certain valve-tray designs and contact tray assemblies and methods. In the contact tray assemblies and the valve designs, it may be seen that the individual valves whether fixed or floating, are illustrated in the drawings with solid surfaces. In other words, both the front and rear legs, as well as the top surface of the valves, whether floating or fixed, are shown to be of solid construction. Other contact-tray valve assemblies are set forth and shown in U.S. Pat. Nos. 6,145,816; 5,911,922; 5,762,834; and 6,089,550. Each of these patents further illustrate aspects of contact tray assemblies and methods as well as valve designs. Additional patents which should likewise be reviewed relative to contact trays include the following four patents in which the Applicant hereof, Michael J. Binkley, is a co-inventor and include: U.S. Pat. Nos. 5,453,222; 4,956,127; 5,106,556; and 5,192,466. The above-referenced patents and statements with regard to the related art are set forth for purposes of understanding the intricacies of the design considerations in contact-tray assembly and method configurations. It would be an advantage to provide a method of and apparatus for enhanced fluid flow manifesting increased efficiency with a valve design having a deflection tab in either a fixed or floating configuration relative to the tray for enhanced fluid interaction.